The present invention is directed generally to asphalt pavers of the floating screed type as typically reflected in, for example, the Donald R. Davin, et al. U.S. Pat. No. 3,776,326. Such floating screed asphalt pavers typically include a self-propelled paving vehicle provided at its forward end with a hopper of limited capacity for receiving paving material directly from a truck. In typical operation, the paver vehicle engages and pushes the truck forwardly along the highway bed, while the truck progressively discharges its contents of paving material into the hopper. A controllable conveyor arrangement transfers paving material from the hopper to the rear of the paver, where it is discharged onto the road bed, in front of transversely disposed screw auger distributors. The augers spread the material laterally in front of a screed, which functions to lay a "mat" of paving material at a uniform level, and also compresses the paving material and smooths the mat surface. Commonly, the screed is arranged and operated as a so-called floating screed, being connected to the paving vehicle only by elongated tow bars. Relatively accurate control over the mat surface is enabled by controlling the height of the tow points, at each side of the paver, and also by controlling the angle of attack of the screed surface relative to the desired final surface of the paving mat.
A standard asphalt paver is constructed with a screed of standard width. Typically, this may be on the order of eight feet. However, the screeds are constructed to accommodate the mounting at one or both ends of screed extension units enabling substantially wider screeds to be assembled. The Donald R. Davin U.S. Pat. No. 3,702,578 illustrates an advantageous form of such screed extension.
For continuous down-the-road paving operations, it is convenient to assemble the paver screed with the desired number and configuration of extensions, which remain in a permanent configuration throughout a lengthy paving sequence. For many other paving jobs, however, such as driveways, parking lots and the like, the pavement width specifications are not always uniform, and there is need for an ability to vary the width of a screed during the course of a paving operation, without stopping the paver.
Early efforts to provide variability in effective screed width are represented in the Poulsen U.S. Pat. No. 3,572,227 and the Lamb, et al. U.S. Pat. No. 3,957,384. However, both of these devices relate to so-called "strike-off" plates, which are extendable laterally beyond the normal end edges of the paver screed. The strike-off plates, however, function only to scrape or level off the surface of the pavement mat in the extension area and are unable to perform the important functions of a screed, which include "ironing out" the mat surface and also providing a degree of initial compression or compaction of the paving material in advance of rolling. Thus, when equipment such as reflected in the Poulsen and/or Lamb, et al. patents is utilized, the character of the pavement is noticeably different in the area of the strike-off extensions than in the area of the screed proper.
More recently, efforts have been made to construct the floating screed of an asphalt paver with adjustable screed extension units which function not only to level off the material, as in the case of the adjustable strike-off units, but also to smooth out and compact the material, as is the typical function of a paving screed. Representative of these efforts are the Schrader U.S. Pat. No. 3,992,124 and the Ruge U.S. Pat. No. 4,129,399. While these screed extension arrangements represent a significant improvement over the use of adjustable strike-off plates, both arrangements still possess serious functional shortcomings, in that the extension elements of the screed are mounted behind the main screed. The effect of this is that, although outward adjustment of the screed extensions is readily accomplished, full inward retraction of the extensions during paving is not possible. This is because of the fact that a substantial body of the paving material becomes trapped in the space between the ends of the main screed and so-called edger plates, which are mounted at the outer extremities of the screed extensions and extend forward therefrom. When the screen extensions are retracted, paving material becomes trapped in this area and blocks full retraction. In practice, this has necessitated stopping of the paving equipment and a manual shovelling out of the paving material trapped by the retracting extension units. This is of course undesirable, not only from the standpoint of the time lost in stopping the paving equipment, but also in the quality of the paving, inasmuch as the floatation of the screed is affected by the speed of its forward movement. When such forward movement stops, the screed may tend to sink into the paving mat, leaving an irregularity in the finished paving surface.
In accordance with the present invention, a novel and improved form of adjustable paving screed is provided for a floating screed asphalt paver, in which the adjustable screed elements are mounted in front of, rather than behind, the main paver screed. The screed extension units, unlike strike-off extensions, are provided with bottom plates of substantial front-to-back dimension, capable of effectively smoothing and partially compacting the paving material, as well as leveling the material at a desired height. In one advantageous form of the invention, a strike-off plate is provided directly in front of the screed extensions and serves, when the extension units are partially extended, to limit the height of the paving material mass in the area between the extended screed units and the front of the primary screed. Accordingly, when the extension units are retracted during paving operations, the opportunity for excessive accumulation of paving material in the area between the closing extension units is limited. By a slight upward angling of the inner edge extremities of the extension units, whatever minimal accumulation of paving material exists is readily plowed under by the screed bottom surface and spread and flattened by the main screed.
In another advantageous form of the invention, the adjustable extension units are designed for operation without the use of a strike-off plate mounted in front thereof. In this modified form of the invention, the adjustable strike-off extension units are designed to provide upwardly angled regions of the bottom surface, and also inwardly and rearwardly angled front surface portions, along the inner edges. Accordingly, as the extension units are retracted during forward paving motion, accumulated paving material is displaced forwardly and/or plowed under the screed extensions, to permit full closing of the extension units without stopping of the paver.
Certain more specific aspects of the invention involve novel and improved arrangements for mounting of the screed extensions on the front of the main paver screed in a manner which, in general, protects the mounting and guiding means from the hot asphalt ambient, and also simplified and easily operated arrangements for vertical and angular adjustment of the screed extensions in relation to the main screed unit.
The apparatus of the invention also includes a novel and simplified, yet efficient means for utilizing exhaust gases from the main screed heater facility for preheating of the screed extension units. In this respect, it is generally desirable to preheat the screed before commencement of paving, after which it is usually adequate to rely upon the contained heat of the asphalt paving material to maintain a desired temperature equilibrium of the screed. In the apparatus of the present invention, the screed extensions are arranged to receive and utilize exhaust heating gases from a conventional screed heater to effect preliminary heating of the extension units without adding consequential cost to the heating system.
For a more complete understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of preferred embodiments of the invention and to the accompanying drawings.